1. Field of the Invention
The present disclosure relates to an oxide thin film transistor and its fabrication method, and more particularly, to an oxide thin film transistor having a bottom gate structure using amorphous zinc oxide-based semiconductor as an active layer and its fabrication method.
2. Discussion of the Related Art
As consumer interest in information displays is growing and the demand for portable (mobile) information devices is increasing, research and commercialization of light and thin flat panel displays (“FPD”), which substitute cathode ray tubes (CRTs), the conventional display devices, has increased. Among FPDs, the liquid crystal display (“LCD”) is a device for displaying images by using optical anisotropy of liquid crystal. LCD devices exhibit excellent resolution, color display and picture quality, so they are commonly used for notebook computers or desktop monitors, and the like.
The LCD includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
An active matrix (AM) driving method commonly used for the LCD is a method in which liquid crystal molecules in a pixel part are driven by using amorphous silicon thin film transistors (a-Si TFTs) as switching elements.
The structure of a general LCD will now be described in detail with reference to FIG. 1.
FIG. 1 is an exploded perspective view showing a general LCD device.
As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for dividing the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying voltage to the liquid crystal layer 30.
The array substrate 10 includes gate lines 16 and data lines 17 which are arranged vertically and horizontally to define a plurality of pixel areas (P), TFTs (T), switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel areas (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant (not shown) formed at an edge of an image display region to form a liquid crystal panel, and the attachment of the color filter substrates 5 and the array substrate 10 is made by an attachment key formed on the color filter substrate 5 or the array substrate 10.
The above-described LCD is a display device receiving much attention so far with its advantages of being light and consuming a small amount of power. However, it is a light receiving device, not a light emitting device, and has a technical limitation with respect to brightness, a contrast ratio, a viewing angle, and the like. Thus, development of a new display device that can overcome such shortcomings is actively ongoing.
An organic light emitting diode (OLED), one of new flat panel display devices, is self-emissive, having good viewing angle and contrast ratio compared with the LCD. Because it does not need a backlight, it can be formed to be lighter and thinner, and is advantageous in terms of power consumption. Also, the OLED is driven at a low DC voltage and has a fast response speed. In particular, the OLED is advantageous in terms of fabrication costs.
Research for a large-scale organic light emitting display is actively ongoing, and development of a transistor achieving a stable operation and durability by securing constant current characteristics as a driving transistor of the OLED is required.
The amorphous silicon thin film transistor used for the above-described LCD may be fabricated in a low temperature process, but it mobility is very small and does not satisfy constant current bias conditions. Meanwhile, a polycrystalline silicon thin film transistor has high mobility and satisfies the constant current bias conditions, but it is difficult to secure uniform characteristics, so it is difficult to increase in area and a high temperature process is required.
Thus, an oxide semiconductor thin film transistor including an active layer as oxide semiconductor has been developed, but application of the oxide semiconductor to the thin film transistor of the conventional bottom gate structure causes degeneration of the oxide semiconductor during an etching process of source and drain electrodes.
FIG. 2 is a sectional view schematically showing the structure of a general oxide thin film transistor.
As illustrated, a gate electrode 21 and a gate insulating layer 15 are formed on the oxide thin film transistor of the bottom gate structure, and an active layer 24 formed of oxide semiconductor is formed on the gate insulating layer 15.
Thereafter, source and drain electrodes 22 and 23 are formed on the active layer 24, and at this time, in the process of depositing and etching the source and drain electrodes 22 and 23, the lower active layer 24 (in particular, a portion ‘A’) is possibly damaged to be degenerated, deteriorating the reliability of the device.
Namely, when the source and drain electrodes are formed according to a wet etching, the active layer is lost or damaged due to the physical properties of the oxide semiconductor which is weak to an etchant. Also, even when the source and drain electrodes are formed according to a dry etching, back sputtering or oxygen deficiency of the oxide semiconductor causes the active layer to be degenerated.
Although not shown, a protection layer (passivation layer) made of silicon oxide film is formed by using a plasma enhanced chemical vapor deposition (PECVD) equipment, and in this case, in the oxide semiconductor constituting the active layer, hydrogen atoms serve as a carrier within the semiconductor thin film by a reaction with H2 gas while the PECVD silicon oxide film is deposited, making the oxide semiconductor changed to a conductor.
Thus, in place of the single protection layer of the silicon oxide film, a dual structure in which an etch stopper that restrains a reaction with H2 gas is additionally formed on the active layer is employed, which, however, has a complicated process and incurs high cost. In addition, when the protection layer of the dual structure is formed by adjusting a rate of flow of the H2 gas of the reaction gas, particles are generated due to precipitation of silicon atoms, disadvantageously narrowing a process window.